WHAT A PARTY! Gov. Paterson yesterday as he dropped in on Dem rival Andrew Cuomo's fund-raiser. Photo: Photos: Christopher Sadowski

Surprise!

Gov. Paterson made an attention-grabbing appearance last night at a birthday-bash fund-raiser for his political rival, state Attorney General Andrew Cuomo — the likely 2010 Democratic nominee for governor over the poll-challenged incumbent.

“I’m just here to wish Andrew a merry Christmas and a happy birthday,” a clearly amused Paterson told The Post as he left after the brief stop-in.

Paterson had been invited to the Cuomo bash at the Sheraton New York in Midtown for “Andrew Cuomo 2010,” the AG’s re-election fund, which doesn’t specify what office he’s running for.

But he was never expected to actually show.

The event was tied to the next campaign-finance filing deadline, on Jan. 15, when Cuomo is expected to have as much as four times as much money as Paterson.

But the governor created his own splash, strolling into the fund-raiser after attending a labor event at the same hotel.

He said Cuomo, who turned 52 on Dec. 6, “thanked me for being here . . . I hope he raised as much money as he wants to.”

“There’s no reason we shouldn’t be wishing him well,” added Paterson, whom the White House has asked to step out of the 2010 race.

Aides to Cuomo noted that Paterson was invited. But attendees at the 1,000-person event — where people ponied up $1,000 a head to see Bon Jovi play — said their jaws dropped.

“Andrew looked shocked,” said one guest. “The governor bum-rushed him. They greeted each other. It was stiff and cordial. It was like seeing your old girlfriend at the prom.”

“Right now, people are closeted Cuomo fans,” said one Democratic insider, adding that Paterson probably went “out of curiosity” to find out who was there — and some guests, indeed, said they were uncomfortable about that.

“David is desperate. He’s calling up everybody looking for money,” said a longtime Paterson supporter.

A Cuomo fund-raiser agreed: “They’re not even in the same ballpark.”

For his part, Cuomo sounded every bit the candidate — noting that Wall Street drives the state’s economy and saying, “We’re not going to be able to raise taxes. People will vote with their feet, I’m sure.”

A Paterson booster insisted the Sheraton stunt “lets everyone know who is still governor. And he ain’t going away.”